Bearing inner rings and bearing outer rings of a rolling-element bearing generally each include a raceway along which rolling elements are configured to roll. The bearing rings can include axially disposed flanges, so-called retaining and/or guide flanges. The retaining flange plays a role in supporting the rolling elements during bearing installation and/or during transport, and the guide flange serves for axially guiding rolling elements, in particular during bearing operation. The retaining flanges and/or guide flanges are disposed axially adjacent to the raceway of the bearing ring and delimit the axial extension of the raceway. In a cylindrical roller bearing the rolling elements are configured as cylindrical rolling elements, and in a tapered roller bearing the rolling elements are configured as truncated-cone-shaped or conical rolling elements. The retaining flange and/or guide flanges serve in these cases for axially supporting the rolling elements, which are supported at their axial end sides on the respective facing flange. In the following discussion, the disclosure relates to bearing rings that are formed one-part with respect to their entire circumference.
It is known to induction harden the raceways of bearing rings in order to make them sufficiently hard and strong so as to adequately withstand mechanical stresses, such as, for example, deformations due to tension or pressure. The induction hardening as a method for setting certain surface layer conditions is based on a structural transformation that results from material-specific thermal treatments. The process can generally be divided into heating a region to a temperature above the austenitizing temperature and, optionally, a subsequently quenching the material to a temperature below the temperature at which martensite starts to form. This produces an austenitic structure in an outer layer of a bearing ring, at a raceway region, for example, while inward of the raceway the initial microstructure remains. Induction hardening is used increasingly often because it allows only part of a surface to be hardened and provides good control of hardness values and hardness curves.
Disadvantageously, during the induction hardening process a thermal expansion of the material occurs that is particularly critical in the region of the bearing retaining flange. This is because, as a rule, the retaining flange and/or the bearing-ring shoulder adjacent to the retaining flange are not hardened and are therefore not heated. Particularly large stresses thus may occur in this region which can lead to undesirable crack formation. Crack formation is particularly problematic in bearing rings of tapered roller bearings because they have a highly varying cross-section over their width with respect to the material thickness, and thus the induction hardening process produces an amount of heat in the rings that varies greatly. The greater the angle that the raceway forms with respect to the rotational axis, the more the material thickness increases over the width of the ring.